What do optogenetics use to excite neurons by light?
Optogenetics: Light to control gene activity?
Optogenetics uses microbial opsins (light sensitive molecules) to excite or inhibit neurons by light
- Each opsin responds to light of particular wavelengths
- Adeno-associated virus (AAV) is used to introduce microbial opsins (required genetic material)
- Cell-specific promotor sequence can restrict opsin expression to particular cell type
What responds to blue and yellow light respectively? What happens?
Blue Light
Channelrhodopsin-2 (ChR2) pump
- Allows positive ions into the cell
- Depolarization
- Neural excitation
Yellow light
Halorhodopsin (NpHR) pump
- Allows negative ions into the cell
- Hyperpolarization
- Neuronal inhibition
How is light delivered in optogenetics? What is the key question?
- Light stimulating a particular rhodopsin is delivered through an implanted optic fibre
- What behaviours are evoked or suppressed when particular cells are excited or inhibited?
What happened to rats with stimulated chR2 in mouse amygdala cells?
ChR2 - excitatory
Induced a predatory response
What does an optogenetic system with CRISPR-Gas9 system allow?
- Light-controlled protein transcription
- Light-controlled genome editing
From genes, to brain, to behaviour: What 2 aspects were discusssed?
- ) Animal Models
- ) Imaging Genetics
What is bioinformatics and what is the advantage?
Genetic associations with behaviour can only hint at the biological pathways involved, unfortunately
- Bioinformatics
- ‘Translate’ a human genetic mutation to target a homologous (similar) gene in a model organism
- Use model organisms for direct measurement at multiple levels
- Structure
- Function
- Behaviour
Which gene ecode serotonin? And where is the SNP of interest located?
- HTR1A is the human gene encoding the serotonin-1A receptor
- The SNP rs6295 is in the promotor region of HTR1A, affecting protein transcription instead of structur
Connect genes to behaviour
- SNP rs6295 in the promotor region of HTR1A is associated with trait anxiety
- Individuals with 1/2 copies of the G allele showed lower trait anxiety than individuals homozygous for C allele
- GG/GC < CC
- Individuals with 1/2 copies of the G allele showed lower trait anxiety than individuals homozygous for C allele
Connect genes to anatomy. (Includes how it does it)
- HTR1A SNP (rs6295) associated with serotonin-binding potential
- Additional copies of the G allele were associated with increased binding potential
- G allele impairs transcriptional repression of the promoter
- Thereby, increasing expression of the 5-HT1A receptor
How is binding potential measured?
Positron emission tomography (PET)
- Detects radiation from an injected tracer with a high affinity for 5-HT1A receptors
- Measures binding potential, infer presence of serotonin-1A receptors
- Higher density of receptors leads to higher binding potential
What kind of receptor is serotonin-1A?
- Autoreceptor
- Located at the presynaptic membrane, where it binds neurotransmitters released by the neuron
- Regulates NT release
Connect anatomy to brain function.
- Reduced capacity for regulation of 5-HT release is associated with increased amygdala reactivity
- Amygdala reactivity = Measured by comparing BOLD activation in the amygdala between two tasks
- Matching angry or fearful faces vs. matching geometric shapes
Connect function to behaviour.
- Increased amygdala reactivity is associated
with increased trait anxiety- Amygdala reactivity was measured by comparing amygdala BOLD activation when viewing fearful vs. neutral faces
Connect genes to function.
- HTR1A SNP (rs6295) is associated with amygdala reactivity
- Additional copies of the G allele were associated with decreased reactivity
Connect anatomy to behaviour.
Increased serotonin-binding potential throughout the brain is associated with lower anxiety
Describne Genes > Brain > Behaviour.
Genes: rs6295 G allele (-) (+)
Brain Structure: 5-HT binding (low)(high)
Brain Function: Amygdala Reactivity (high)(low)
Behaviour: Trait Anxiety (high)(low)
By comparing measurements across multiple levels, we can…?
We can reveal the likely biological pathway linking a genetic variant to a behavioural phenotype
Genes > Brain > Behaviour, explain the full process in terms of presence?
- Presence of G allele of rs6295 impairs repression of HTR1A transcription
- Increased receptor density
- indexed by binding potential
- Decreased amygdala reactivity
- Predisposes to low anxiety
Genes > Brain > Behaviour, explain the full process in terms of absence?
- Absence of the G allele of rs6295 promotes repression of HTR1A transcription
- Decreased receptor density,
- indexed by binding potential
- Increased amygdala reactivity
- Predisposes to high anxiety
